Thin films of materials are used for many applications. In some of these applications the films or layers deposited on substrates are not electrically active. For example, hard ceramic coatings such as carbides, nitrides and oxides of various elements are used to provide wear resistance to the substrate that they are deposited on. Some coatings provide color. Some others, such as tribological coatings, reduce the friction coefficient on the surface of the substrate that they are deposited on. In applications where the thin film is electrically active, the electrical activity varies depending on the nature of the layer. For example, metallic layers such as Al, Cu, Ni etc. may act as conductors. Semiconducting layers, on the other hand may be used in thin film device fabrication such as thin film transistors, solar cells, photoconductors, detectors etc.
Thin films may be deposited on substrates by a large variety of techniques, including vacuum evaporation, sputtering, chemical vapor deposition, electrodeposition, electroless plating, ink deposition, melt deposition, dipping, spinning etc. The mechanical, structural and/or electrical properties of the deposited films generally vary with deposition conditions including the temperature of the substrate during the film deposition step. Therefore, the substrate temperature needs to be closely controlled during the deposition of the films.
In some compound layer growth techniques the film deposition involves more than one step. As will be described further below, in two-stage or multi-stage processes, certain elements of the desired compound may first be deposited on a substrate in the form of a precursor layer and then reacted to form the desired compound film. In these cases, the substrate temperature need to be closely controlled, especially during the reaction step because the properties of the resulting compound layer depend, to a large extent, to details of the reaction process.
In processes involving heating the substrate during film deposition, various designs of substrate heaters are employed. Typically the substrate to be coated with the film is placed on or brought to proximity of a substrate heater, which may be, for example a lamp heater or a resistive heater. In applications where deposited layers are reacted to form compounds, reactions may be carried out in ovens, furnaces, rapid thermal process tools etc, where various different temperature control means are utilized. In certain applications, however, very fast response times are required for heating and/or cooling the substrates on which the films are grown. Additionally, it is always attractive to apply the heat mostly to the substrate but not to the environment the substrate is placed in.
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k (where 0≦x≦1, 0≦y≦1 and k is approximately 2) have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
One technique for growing Cu(In,Ga)(S,Se)2 type compound thin films for solar cell applications is a two-stage process where at least two components of the Cu(In,Ga)(S,Se)2 material are first deposited onto a substrate, and then reacted with each other and/or with a reactive atmosphere in a high temperature annealing process. For example, for CuInSe2 growth, thin layers of Cu and In are first deposited on a substrate and then this stacked precursor layer is reacted with Se at elevated temperature. If the reaction atmosphere also contains sulfur, then a CuIn(S,Se)2 layer can be grown. Addition of Ga in the precursor layer allows the growth of a Cu(In,Ga)(S,Se)2 absorber.
There are many other versions of the two-stage process that have been employed by different research groups. For example, stacked layers of sputter deposited (Cu—Ga)/In, and co-evaporated (In—Ga—Se)/(Cu—Se), and (In—Ga—Se)/Cu stacks have all been used as precursor materials which were reacted at high temperatures with S and/or Se to form the final absorber film. Sputtering and evaporation techniques have been used in prior art approaches to deposit the layers containing the Group IB and Group IIIA components of the precursor stacks. In the case of CuInSe2 growth, for example, Cu and In layers were sequentially sputter-deposited on substrates and then the stacked film was heated in the presence of gas containing Se at elevated temperature, as described in U.S. Pat. No. 4,798,660. More recently U.S. Pat. No. 6,048,442 disclosed a method comprising sputter-depositing a stacked precursor film comprising a copper-gallium alloy layer and an indium layer on a metallic back electrode layer or a contact layer and then reacting this precursor film with one of selenium and sulfur to form the absorber layer. U.S. Pat. No. 6,092,669 described sputtering-based equipment for producing such absorber layers. Electrodeposition may also be used to deposit the metallic precursors, such as Cu and In layers, on a substrate.
Irrespective of the technique used for the deposition of the precursor layers comprising at least one Group IB material and at least one Group IIIA material, the precursor layer need to go through a reaction step to form the Group IBIIIAVIA compound layer. This is achieved heating the substrate on which the precursor is deposited and exposing it to at least one Group VIA material. This reaction step may be carried out in batch furnaces where a large number of substrates are heated up slowly to elevated substrates of 400-550 C in the presence of Se and/or S. This batch process is typically carried out for 20-60 minutes. One other approach utilizes a rapid thermal process where, typically, precursors comprising Group IB, Group IIIA and Group VIA materials are heated up rapidly to initiate and carry out reaction between them forming the Group IBIIIAVIA compound layer. It is reported that ramp rates in such a process is very important and that temperature rise rates of about 10 C/sec is needed to avoid defects in the forming compound layer.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a substrate 11, such as of a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over a conductive layer 13 or a contact layer, which is previously deposited on the substrate 11 and which acts as the ohmic contact to the device. Various conductive layers comprising Mo, Ta, W, Ti, stainless steel etc. have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use a conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. The preferred electrical type of the absorber film 12 is p-type, and the preferred electrical type of the transparent layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of FIG. 1 is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the superstrate side.
In any process involving heating a substrate on which a thin film material such as a thin film semiconductor is being formed, heating uniformity, heating rate and sometimes cooling rate are important parameters that influence the properties of the film that is formed on the substrate. There is, therefore, a need to develop novel approaches to closely control such parameters.